I had this too. I was very tomboy-ish growing up, people used to compliment my mum on her her 'two boys' (me and my brother)! I had a little wobble when I found out I was having a girl, as I had a very sudden and - as others have said, very stereotypical and cliched - flash of ballet and french plaits, and that scared me.
But as others have already said, you are having a baby, not a gender. Your baby is their own person, not a 'girl' or a 'boy', though before you have a baby it is very easy to think of them in a gender-orientated way. We're 18m in and the only "girls" things I have done so far are learning to wipe front-to-back rather than tucking a willy inside a nappy (only had experience with boy babies!) and learning to tie her hair back, as she has loads of hair, more than me already. She is just starting to get interested in dolls and cuddly toys, but I suspect no more than some boys might do at her age. She is showing a fascinating mix of traditional/stereotypical boy and girl behaviour, hugging dolls one minute, dragging older boys out of toy cars the next.
What I am trying to say is that by the time they possible, maybe might start showing an interest in the things that you didn't do as a child, you are so comfortable with them as the person they are, it doesn't matter.
Plus, a boy might not have the same interests as you did as a child anyway!